Hi, my name is Nick Stasulis and I am a hydrologic technician with the Maine Office of the New
England Water Science Center.
In this video, we will briefly review the midsection method and some considerations
when using this method with an ADCP.
The most practical way to measure a stream discharge with a point velocity meter is the
use of the velocity-area method, which requires two pieces of information, the area of the
cross section and the water velocity through that cross section.
The discharge is determined as the product of those two variables.
In the USGS, we use the midsection velocity-area method.
In the past, this method was used with point velocity meters, but this same method can
be used with ADCPs, as we’ll discuss.
With the midsection method, we divide the channel into a number of sections, or stations,
ideally 20 to 30, or enough to characterize the channel geometry.
We determine the area of each section by measuring it’s depth and width, and then use the resulting
area and velocity to compute discharge.
The depth will be determined by the ADCP.
With a point velocity meter, velocity readings are collected at discrete points, some combination
of 0.2, 0.6 or 0.8 of the depth, and a vertical velocity curve used to determine a mean velocity
for the station.
With an ADCP, a profile is measured, extending from very near the surface to very near the
bottom, and after using extrapolation to determine velocity in the unmeasured areas, a mean velocity
is determined for the station.
Notice the depth and velocity at each station extends halfway to the next and previous stations.
When selecting the location for these stations, we need to ensure that no station has more
than 10% of the total flow and that ideally, no station has more than 5% of the total flow,
including the edge sections.
Using this general criteria, sections should be closer together in parts of the cross section
with greater depths and velocities.
Once all of the stations are collected, we sum the discharges for each and the result
is the measurement discharge.
Now, let’s highlight a few important considerations with this method.
The most important aspect of the midsection method is the cross section you select.
When using an ADCP, you might be limited to bridges, manned cableways or bank operated
cableways, but you might be able to utilize temporary lines or pulley setups to optimize
your cross section.
The reach should be straight and uniform, allowing for steady and uniform velocity through
the cross section, and the streambed should be free of rocks, weeds, or large obstructions.
For more detail on the midsection method, you can review the online training found at
the link shown (http://training.usgs.gov/TEL/Nolan/SWProcedures/MidSectionMeasurements/PresenterMaterial/index.htm).
With ADCPs, you still need to measure the channel width and enter the station location
into the manufacturer software.
The width could be measured from a cableway marking, bridge marking, tagline, or other
method, but it’s necessary to measure regardless.
Also, you need to make sure that you are measuring and entering the angle of the flow through
the cross section, something that is covered in detail in later videos.
If you are familiar with point velocity meter midsection measurements, such as with a FlowTracker
or Price meter, you will notice that the midsection method with ADCPs is not much different.
We will still pick a good cross section, measure our distance moved across the channel, and
compute a velocity at each station.
The biggest difference with an ADCP is that we collect a profile at each station, not
a point velocity, and extrapolation methods play a part in that.
We will discuss extrapolation in a later video.
Another important point is that ADCPs typically should not use bottom track with the midsection
method as it’s assumed the ADCP is stationary during data collection, so we don’t need
to worry about moving bed biases or bottom tracking issues.
The ADCP’s beams are still being used to measure depth, however.
This video highlighted some main points of the midsection method as it’s used with
ADCPs.
Other videos will cover the process is more detail, for each ADCP manufacturer.